1. Field of the Invention
This invention relates to a transporting system driving method using linear induction motors which ensures smooth drive control.
2. Description of the Prior Art
As compared with a rotary type induction motor, a linear induction motor has a large exciting current and a low power factor and efficiency. Therefore, the linear induction motor is rarely used as a driving apparatus for a continuous running system.
Generally, in a transporting apparatus there exists a large difference between the thrust required upon starting and acceleration at full-load and the thrust required during regular speed at non-load. In contrast to the rotary type induction motor, the linear induction motor adopts a direct driving mechanism which cannot use a speed reducer and therefore is required to directly generate high thrust. Thus, most linear induction motors have a large size and capacity, and a system of plural dispersed linear induction motors is usually adopted.
In the linear induction motor, it is impossible to adjust the running speed of a movable body having a secondary conductor using a speed reducer, and therefore adjustment of the running speed is effected by a change of voltage and frequency impressed upon the linear induction motor. A description of such adjustment is made below with reference to FIG. 6-FIG. 8. FIG. 6 shows a speed-thrust characteristic which varies with the frequency impressed upon the linear induction motor. In FIG. 6, each of f.sub.1, f.sub.2 . . . . . f.sub.5 denotes a frequency, in which f.sub.1 &gt;f.sub.2 &gt;f.sub.3 &gt;f.sub.4 &gt;f.sub.5.
FIG. 7 shows a system for continuously driving movable bodies W, each having a secondary winding, by a plurality of primary windings LIM1, LIM2 . . . . . LIMn arranged on the ground (including floor surface and table). Movable bodies W are connected in an endless path and each primary winding (hereinafter referred to as a linear induction motor) is connected to a common inverter INV. The CPU (control mechanism) causes the inverter INV to impress signals having a certain frequency and voltage upon the linear induction motors LIM1-LIMn on the basis of speed signals obtained from a movable body running speed detector D.
FIG. 8 shows the main point of the operation mentioned above. Upon starting, in order to generate acceleration exceeding the required starting thrust Fst, a frequency in which thrust characteristic F1a is obtained is generated by the inverter INV and thus starting and acceleration are carried out, whereby movement of the movable bodies starts and is accelerated. When a movable body W has reached the rated speed VR as indicated by a detecting signal from the speed detector D, the CPU gives an instruction to the inverter INV to reduce the frequency and thus the thrust curve F2a which ensures the required running thrust FR is obtained and stable running is carried out.
In the case where an increase in load or change of speed is required during running, the inverter INV generates the required frequency and voltage in accordance with instructions from the CPU, from which each of the linear induction motors LIM1-LIMn generates the required thrust.
In the above case, the output of the linear induction motor during light load running reduces in comparison with the highest thrust (as in the case of the rotary type induction motor), but since the linear induction motor requires a large exciting current, current values are almost constant even when an output change is generated resulting from the increase or decrease in load. Due to the change of the phase difference (power factor) between voltage and current, the effective electric power increases or decreases with the load, but the apparent electric power (KVA) does not change substantially.
Accordingly, in the continuous driving apparatus using plural linear induction motors, if all of the linear induction motors are kept excited at light load, the power source burden resulting from a comparatively large exciting current remains large and a heat loss caused by the coil resistance of the linear induction motor is not changed much. Moreover, from the characteristic of the linear induction motor (lowering of efficiency at light load), the conventional linear induction motor is not recommendable for high efficiency running and requires an inverter of large capacity.